The preparation of alkylphosphonic acid dialkyl esters of the formula RP(O)(OR').sub.2 by the reaction of alkylhalides of the formula RX with dialkylphosphite salts of the formula (R'O).sub.2 POMe in alcoholic solution of the formula R'OH according to the following reaction scheme: ##STR1## has been known since the turn of the century and is known as the "Michaelis-Becker" reaction. See, for example, Houben-Weyl "Methoden der organischen Chemie", 4th edition (1963), Vol. 12, pages 446 et seq. Preferably R represents n-alkyl in this reaction; the yields are lower with secondary and tertiary alkylhalides. The X group is preferably chlorine, but it may also be bromine or iodine. For technical reactions, R' is preferably ethyl or n-butyl, and Me is preferably sodium. Furthermore, while the reaction is preferably carried out in alcohols, ethers or aromatic compounds may optionally be used as solvents.
The principle of the Michaelis-Becker reaction has also been applied to the synthesis of oligophosphonic acids. For example, when allylbromide (CH.sub.2 .dbd.CH--CH.sub.2 Br) is used, substitution takes place, followed by the addition of sodium dialkylphosphite. Finally, the diphosphonic acid ester of the formula ##STR2## is obtained. The conversion of .alpha.-bromostyrene to the ester of the formula ##STR3## proceeds in a similar manner.
No mention is made in the literature about successful conversions of polyhalogenated olefins under the conditions of the Michaelis-Becker reaction.
U.S. Pat. No. 3,471,552 discloses a process by which methane polyhalides such as chloroform, carbon tetrachloride, bromoform, or carbon tetrabromide are reacted with sodium dialkylphosphite, a mixture of xylene and tetrahydrofuran being used as solvents. Methane oligophosphonic acid esters are reportedly formed. However, testing of the data disclosed shows that the reactions do not proceed as smoothly as stated. Side reactions to the planned reactions are dominant, as can be demonstrated with nuclear resonance spectra. It has to be concluded that the reactions of chloroform and, especially, carbon tetrachloride are described incorrectly in this patent.
When the reaction of a polyhalogenated olefin selected from the group consisting of vinylidene chloride (CH.sub.2 .dbd.CCl.sub.2), trichloroethylene (CHCl.dbd.CCl.sub.2), and tetrachloroethylene (CCl.sub.2 .dbd.CCl.sub.2) with sodium diethylphosphite in ethanol is attempted, the following observation is made: Hardly any conversion takes place in the case of vinylidene chloride and trichloroethylene, and tetrachloroethylene converts quantitatively into the triethyl phosphate. Polyphosphonic acid compounds are not produced.